This invention is related generally to gas-turbine power plants, and more specifically relates to gas-turbine generation plants with increased output at high ambient temperature conditions, and is especially suited for single-shaft gas turbines.
Governments and industries from around the world have spent tens of billions of dollars in development and refinement of gas turbine power plants over the last 60 years. In addition to their widespread use in military and civilian aviation, gas turbines are an increasingly important factor in electric power generation. In 1999 sales of gas turbines for the electric power market amounted to 18 billion dollars. Gas turbines and combined-cycle plants now dominate the world market for new generation capacity.
Yet despite this tremendous investment in engineering and construction related to gas-turbine power plants, there is a basic mismatch between the capability of the turbine and the needs of the electrical power grid. A major problem is that gas-turbine capacity decreases as inlet air temperature increases, while power demand increases at high ambient temperatures. This situation means that the capability of a gas-turbine power plant is lowest at precisely the conditions when the electricity is most needed.
FIG. 1 shows how inlet air temperature affects capacity for a typical gas turbine. The normal rating condition is 59xc2x0 F. and the capacity decreases at a rate of approximately 0.4% per xc2x0 F.
The conventional approach used by designers of power plants for sizing generators, transformers, power distribution equipment, and other auxiliary equipment is based on achieving maximum turbine output at low ambient air temperatures. For some turbines, the generator is sized for the lowest available ambient air, which can be xe2x88x9220xc2x0 F. or colder. For others, heaters or other means are used to keep the inlet air temperature at about 35xc2x0 F. to 40xc2x0 F. to prevent ice build up inside the compressor section of the gas turbine.
The problem with this approach is that the generator and associated auxiliary equipment will output maximum power at low ambient air temperatures, while the peak generating requirement occurs at high ambient temperatures. With the advent of a competitive generating industry with real-time pricing on power generation, the conventional approach results in generator power distribution equipment that is oversized by 20% or more at summer peaking conditions. The result is a significant cost penalty.
In the days of highly regulated electric utilities, this cost penalty was simply passed on to consumers and did not affect utility profit margins. With today""s competitive environment, this cost penalty is no longer acceptable.
Another factor that further aggravates this problem is the increased use of air conditioning. This is especially true for utilities in northern climates, where air conditioning usage was rare. Heat waves in recent years have made air conditioning virtually a necessity and have strained generating capacity in many cities in the northern United States. This situation combined with increasing use of natural gas for heating has created summer peaking problems for practically all utilities in the United States. Increased use of air conditioning is creating similar problems throughout the world.
The prior art has attempted many different approaches for cooling the inlet air to the turbine in order to reduce or eliminate this capacity penalty. A summary of these approaches is described in the ASME paper, xe2x80x9cOptions in Gas Turbine Power Augmentation Using Inlet Air Chillingxe2x80x9d, Igor Ondryas et al, presented at the Gas Turbine and Aeroengine Congress and Exposition, Jun. 11-14, 1990, Brussels, Belgium. Among the known alternatives for cooling are direct and indirect evaporative, electric vapor-compression, absorption, and thermal storage systems.
Of these many alternatives, direct evaporative cooling is the only approach that has seen significant application. Direct evaporative cooling has the advantage of low cost and simplicity, but the ambient wet-bulb temperature limits the possible temperature reduction. For locations in the eastern U.S. direct evaporative cooling can reduce inlet air temperatures by 10 to 20xc2x0 F. Larger reductions are possible in warm, dry climates such as those of the southwestern U.S. While direct evaporative cooling is helpful, it does not allow the turbines to run at their full design capacity. After over 50 years of intensive research and development in gas turbines, no one has produced a better approach for dealing with high ambient temperatures.
Mechanical inlet air chilling is another alternative that has seen limited use. The air is typically chilled to approximately 50 degrees Fahrenheit, which gives a significant increase in capacity at high ambient temperatures. However, relatively low temperatures force the cooling system to cool the inlet below the ambient dewpoint temperature, which results in a large latent cooling load. In addition, the low cooling temperatures reduce the efficiency and capacity of the refrigeration systems used to provide the cooling. These problems increase the cost of these systems, and the large cooling power requirement offsets a significant portion of the extra turbine output realized at high ambient temperatures using this type of cooling system. Mechanical cooling systems have seen only very limited application on turbines as a result of these problems.
Gas-turbine supercharging is another approach for increasing turbine capacity. This approach is described in recent U.S. Pat. No. 5,622,044 and in the paper, xe2x80x9cSupercharging of Gas Turbines by Forced Draft Fans With Evaporative Intercoolingxe2x80x9d by R. W. Foster-Pegg, ASME 1965. These systems use a high-pressure fan to increase the inlet air pressure to a gas turbine, combined with an evaporative cooler downstream of the fan as a way of increasing turbine capacity. This setup can give large capacity advantages, but requires additional components and complexity. In addition, the systems are quite bulky and can take large amounts of space, and designs in the prior art can be expensive for the additional power output. While my copending U.S. application Ser. No. 09/475,154 improves many of the cost and space issues with supercharging, supercharging has only seen very limited commercial application and is not suitable for many installations.
U.S. Pat. No. 5,768,884, entitled xe2x80x9cGas turbine engine having flat rated horsepower,xe2x80x9d describes a multishaft turbine with a variable intercooling system that maintains a constant temperature inlet air stream to a high-pressure compressor for a range of conditions. This system maintains a constant power output over a range of temperatures, but at temperatures below 59xc2x0 F. the intercooler is effectively off and the output increases in response to changes in ambient temperatures. This approach has the disadvantages that the turbine capacity at lower ambient temperature is allowed to rise, which effectively sets the required size for the generator and related components and negates much of the advantage of the system. In addition the system requires a complicated and expensive arrangement of multiple spools and a heat exchanger for the intercooler. This cost and complexity means that it is not suitable for the majority of gas turbines for power generation, which use simple, single-spool configurations.
While many different ways of changing the power output of gas turbines appear in the prior art, they have not been used to address the issue of the sizing of generators at high ambient temperature. Typical approaches for reducing turbine capacity include variation in combustor output, heating inlet air, and recirculation of a portion the compressor output.
Variable compressor speed has also been used as an option for controlling turbine output. For example U.S. Pat. No. 6,003,298, entitled, xe2x80x9cSteam driven variable speed booster compressor for gas turbine,xe2x80x9d describes a multishaft gas turbine combined-cycle plant that uses a variable-speed steam turbine to drive a low-pressure compressor. The change in compressor speed is sufficient to maintain similar turbine operating conditions with the generator running at both 50 and 60 Hz line frequency. Likewise, U.S. Pat. Nos. 4,251,987 and 3,853,432 describe variable-speed compressor arrangements using differential gearing. While these systems show additional ways of varying turbine capacity, none of them addresses the root problems with gas-turbine capacity at high ambient temperatures.
In accordance with the present invention an improved gas-turbine power plant contains a compressor that is sized relative to the turbine section so that a maximum design power output occurs at a high ambient temperature, and a control system that modulates the turbine output to prevent overload at lower ambient temperatures.
Objects and advantages
a) to maximize power output at high ambient temperatures
b) to minimize capital cost requirements
c) to minimize system complexity
d) to ensure reliable operation
e) to minimize design changes required to existing products
f) to allow retrofit of existing turbines